Tactical mission planning for airborne vehicles, e.g. UAV:s or robots, where the missions are carried out on ground targets is a very complex task. Any deviation from navigating along a straight line between two locations may cause a tactical deviation as well as a deviation in navigation performance. When routing a trajectory for a covert operation, it is desirable to route the paths such that maximum terrain masking is achieved. Particularly, it is desirable to avoid highly visible areas such as ridge tops and wide open areas. However, an advantageous route planning from a terrain point of view may cause a tactical disadvantage due to e.g. threat exposure or a disadvantage due to the performance of the navigation systems.
Digital terrain elevation data (DTED) is a digital representation of the topography of a geographical region. DTED may be produced from satellite photographs or other means and may include digital data representing altitudes and type of terrain. The data is preferably represented in a grid with equal spacing between the nodes.
According to the prior art, it is known to route covert flight paths using DTED such that the flight path is always in the lowest local area. This routing method rests on the assumption that the best terrain masking will always be at the lowest local elevation. However, this assumption is not always true, and thus the prior art method frequently results in less than optimum routing of flight paths. A significant disadvantage with this prior art method is also that it does not interrelate the tactical performance with the performance of the navigation system.
Terrain navigation, where the actual position of a vehicle is predicted from terrain information available in the present and preceding environment, is increasingly becoming an integrated function in vehicle navigation systems. This increases the possibility of planning a trajectory in a disturbed environment where GPS is not available and to replan trajectories during a mission. However, replanning of a trajectory during a mission does not only involve replanning from a navigational point, but also from a tactical point of view. Known methods of planning trajectories do not satisfactorily address this issue.
One method of routing a covert flight path is disclosed in U.S. Pat. No. 5,504,686. The disclosed method involves routing a covert flight path from digital terrain elevation data of a geographical region. A cost surface is formed from a weighted combination of hideability and flyability in the given area. The flight path is routed in the areas of the costing surface that are both hideable and flyable. However, this method does not take the performance of the navigational system into the route calculation.
Hence, the prior art includes various solutions for planning trajectories based on terrain data. However, they do not involve the complete functionality of the navigational system in the optimization problem. Maps for terrain navigation may be included, but only to display a static image of where the flyability in the terrain is considered to be low and not to dynamically integrate navigational performance and tactical performance in the optimization of the trajectory. Therefore, the trajectories achieved through the prior art planning systems are far from optimized.